It is known in the art that silsesquioxane and siloxane resins are useful in the electronic and semiconductor fields to coat silicon chips and other similar components. Such coatings protect the surface of substrates and form dielectric layers between electric conductors on integrated circuits. Such coatings can be used as protective coatings, interlevel dielectric layers, doped dielectric layers to produce transistor like devices, pigment loaded binder systems containing silicon to produce capacitor and capacitor like devices, multilayer devices, 3-D devices, silicon on insulator devices, coatings for superconductors, superlattice devices and the like. These resins include hydrogen silsesquioxane resins as well as silsesquioxane resins containing a significant portion of organic moieties.
The production of silsesquioxane resins is well known in the art. For example, Collins et al. (U.S. Pat. No. 3,615,272, issued Oct. 26, 1971) teaches the production of a nearly fully condensed resin which may contain up to 100-300 ppm silanol by a process comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium and then washing the resultant resin with water or aqueous sulfuric acid. Bank et al. (U.S. Pat. No. 5,010,159, issued Apr. 23, 1991) teaches a method comprising hydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysis medium to form a resin which is then contacted with a neutralizing agent. Other hydridosiloxane resins, such as those disclosed in Weiss et al. (U.S. Pat. No. 4,999,397, issued Mar. 12, 1991) are produced by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic hydrolysis medium. The use of hydrogen silsesquioxane resin in forming coatings on electronic devices is also well known in the art.
Coating techniques of general applicability in forming silicon-based coating include deposition techniques and spin coating. Spin coating per se is well known in the art, as is the equipment for such coating and the process conditions employed therein. See, for example, Wolf et al., SILICON PROCESSING FOR THE VLSI ERA, Volume 1 (Process Technology) (Lattice Press, Sunset Beach, Calif. 1986), incorporated herein by reference.
Despite its widespread use, certain undesirable results also accompany spin coating. The surface tension of the film precursor compositions, results in some of the film precursor wicking around to and coating the back side edge of the wafer during the spin-coating process. Also, as the spin-coating process progresses, the film precursor becomes progressively more viscous as the solvent evaporates. This increased viscosity can result in the formation of fine whiskers as the film precursor is spun off the wafer in the latter stages of the process. These whiskers then dry on the edge of the wafer. Moreover, as the film precursor continues to dry and increase in viscosity during the spin-coating process, excess precursor is less likely to leave the wafer and instead builds up as an edge-bead at the outer reaches of the wafer surface.
These coating-related problems can cause significant difficulties in the overall integrated circuit fabrication process. Substances on the backside of the wafer can be deposited elsewhere and cause contamination, and can also prevent the wafer from lying flat on ultraflat surfaces. The roughness on the underside of the wafer can affect a number of subsequent process characteristics, including focus, alignment, planarity, and the like. Whiskers on the wafer edges can easily break off in subsequent processing steps, causing particulate contamination in one or more regions of the manufacturing equipment. Finally, the presence of an edge-bead on the front surface edge produces a distorted surface, which can greatly affect focus, alignment, planarity and the like in later process steps.
The art is aware of the problems associated with residual film precursor at the edges and sides of the wafer, and generally seeks to overcome them by applying a small stream of a solvent for the precursor to the edge of the wafer so as to dissolve and remove the unwanted residue. In many cases, the solvent stream is applied to the backside edge of the wafer and it is permitted to wick around by capillary action to the front edges so as to remove backside edge residue, whiskers and edge bead. With certain newer equipment, it is possible to apply the solvent stream from both front and back sides of the wafer simultaneously. In all cases, the object is to remove from the wafer a strip of film precursor which is adhered to the wafer sides, the back surface outer edges of the wafer, and the outer edges of the front surface of the wafer, to leave as defect-free a film as possible. See, in this regard, Durrant et al., Microcontamination, Apr. 1985, pp. 45-51, incorporated herein by reference.
To effectively coat the film precursors onto a substrate, they are dissolved in a solvent to form coating compositions. Similarly, solvent compositions are known which are used for stripping undesired cured and uncured film precursor from wafers, removing undesired edge bead from spun wafers and cleaning related film deposition processing equipment.
Organic solvents are frequently used for these purposes. The solvents most commonly used include alcohols, methylisobutyl ketone, heptane, dibutyl ether, propylene glycol methyl ether acetate, ethyl acetate and propyl acetate. The use of unadulterated organic solvents with siloxane and silsesquioxane film precursors is problematic. For example, silsesquioxane resins (e.g., hydrogensilsesquioxane resin), tend to gel and polymerize in the presence of both polar (e.g., alcohols) and non-polar (e.g., hexanes, negative resist developer) organic solvents. These solvents are, therefore, less than ideal for use in cup-rinse. line cleaning and edge bead removal applications because film precursor forms gels and particles.
In addition, due to their toxicity, adverse environmental effects and expense of disposal, it is desirable to reduce the amount of volatile organic solvents used in semiconductor device manufacture. The odor of any chemical compound, especially one used in enclosed environments such as a fabrication laboratory for integrated circuits, has become an important selection criterion for all organic solutions. Chemicals introduced into these laboratories are required to have as high a value of odor threshold as possible, that is, maximum amount of solvent vapor in the environment without causing any deleterious health effects.
A method for removing the edge bead and cleaning the components of the spin coating apparatus with a solvent that did not cause gelling and/or polymerization of silsesquioxane and siloxane resins would constitute a significant advance in semiconductor device manufacturing. Moreover, solvents useful for these purposes that were of low volatility and low toxicity would be highly desirable. Quite surprisingly, the present invention provides such a method.